1. Field of the Invention
The invention relates generally to optical sensors and more particularly to optical sensors utilizing lasers and optical fibers.
2. Description of the Prior Art
The development of optical sensors to detect physical fields is being actively pursued in laboratories throughout the world. These sensors can be orders of magnitude more sensitive than the electronic devices they are replacing and have other advantages related to deployment flexibility. One particularly promising type of sensor is the laser sensor.
The laser sensor utilizes the well-known effect that an external feedback mechanism may be utilized to modify the operating characteristics of a laser. For example, Dandridge et al. in an article entitled "Diode Laser Sensor", Elec. Letters., Vol. 16, 1980, pp. 948-9, disclose a laser-sensor with a fixed-reflectivity mirror mounted externally to a laser cavity. This mirror feeds a portion of the laser light output back into the cavity. The fixed-current laser output power is found to vary with the displacement of the mirror along the optical axis. If the mirror is mounted so that its displacement is modulated by a physical field then the parameters of the field may be measured by monitoring the output power of the laser.
The primary disadvantage of the above-described sensor is that it only senses the physical field over a small region of the field. Thus, the laser-sensor is essentially a point sensor.
Sensors for sensing the physical field over an extended region of space have been realized utilizing optical fibers. Typically these fiber-optic sensors include a laser-source a length of optical fiber, and a photodetector. The fiber may be very long to facilitate sensing the physical field over an extended region of space. Additionally, the fiber may be configured to allow spatial shading and directional sensing. For sensitive operation, the phase of the laser light propagating through the fiber must be accurately measured and this is typically accomplished by configuring the fiber as a Mach-Zehnder interferometer. These fiber optic Mach-Zehnder interferometers are complicated structures, however, requiring two precision fiber-optic beamsplitters and precise matching of reference and sensing arm lengths. Additionally, the sensitivity is degraded by noise inherent in the laser beam. The complexity of this arrangement is also increased by the photodetectors required to monitor the intensity of the output beams.